Structures for packaging stress-sensitive micro-electro-mechanical system stacked onto electronic circuit chip

ABSTRACT

A packaged micro-electro-mechanical system (MEMS) device (100) comprises a circuitry chip (101) attached to the pad (110) of a substrate with leads (111), and a MEMS (150) vertically attached to the chip surface by a layer (140) of low modulus silicone compound. On the chip surface, the MEMS device is surrounded by a polyimide ring (130) with a surface phobic to silicone compounds. A dome-shaped glob (160) of cured low modulus silicone material covers the MEMS and the MEMS terminal bonding wire spans (180); the glob is restricted to the chip surface area inside the polyimide ring and has a surface non-adhesive to epoxy-based molding compounds. A package (190) of polymeric molding compound encapsulates the vertical assembly of the glob embedding the MEMS, the circuitry chip, and portions of the substrate; the molding compound is non-adhering to the glob surface yet adhering to all other surfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/266,227, filed Feb. 14, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/867,563, filed Jan. 10, 2018, (now U.S. Pat. No.10,233,074), which is a continuation of patent application Ser. No.15/134,574, filed Apr. 21, 2016 (now U.S. Pat. No. 9,896,330), whichclaims the benefit of Provisional Patent Application No. 62/278,224,filed Jan. 13, 2016, the contents of all of which are hereinincorporated by reference in its entirety.

FIELD

Embodiments of the invention are related in general to the field ofsemiconductor devices and processes, and more specifically to thestructure and fabrication method of a package for a stress-sensitivemicro-electro-mechanical system (MEMS) stacked onto an electronic devicechip.

DESCRIPTION OF RELATED ART

The wide variety of products collectively calledMicro-Electro-Mechanical devices (MEMS) are small, low weight devices onthe micrometer to millimeter scale, which may have sensors ormechanically moving parts, and often movable electrical power suppliesand controls, or they may have parts sensitive to mechanical, thermal,acoustic, or optical energy. MEMS have been developed to sensemechanical, thermal, chemical, radiant, magnetic, and biologicalquantities and inputs, and produce signals as outputs. Because of thesensitive parts and moving parts, MEMS have a need for physical andatmospheric protection. Consequently, MEMS are placed on or in asubstrate and have to be surrounded by a housing or package, which hasto shield the MEMS against ambient and electrical disturbances, andagainst mechanical and thermal stress.

A Micro-Electro-Mechanical System (MEMS) integrates mechanical elements,sensors, actuators, and electronics on a common substrate. Themanufacturing approach of a MEMS aims at using batch fabricationtechniques similar to those used for microelectronics devices. MEMS canthus benefit from mass production and minimized material consumption tolower the manufacturing cost, while trying to exploit thewell-controlled integrated circuit technology. The mechanically movingparts and the electrically active parts of a MEMS are fabricatedtogether with the process flow of the electronic integrated circuit (IC)on a semiconductor chip.

Following the technology trends of miniaturization, integration and costreduction, substrates and boards have recently been developed which canembed and interconnect chips and packages in order to reduce boardspace, thickness, and footprint while increasing power management,electrical performance, and fields of application. Examples includepenetration of integrated boards into the automotive market, wirelessproducts, and industrial applications.

As examples, integration boards have been successfully applied to embedwafer level packages, passives, power chips, stacked and bonded chips,wireless modules, power modules, generally active and passive devicesfor applications requiring miniaturized areas and shrinking thickness.

SUMMARY

A general trend of the electronic industry requires fewer and smallercomponents in a system. In this trend, a technical and market advantagewill be awarded when the number of components in a system is reduced andthe product consumes less space and operating power yet offers improvedelectrical characteristics and higher reliability. For semiconductordevices, a particular advantage can be gained, when a low cost packagingtechnology can be realized so that it promotes miniaturization byintegrating or eliminating parts, and protection by absorbing orshielding disturbances.

As for micro-electro-mechanical systems (MEMS), applicants realized thata particular performance and market advantage can be obtained when aMEMS device can be stacked on an existing electronic circuit devicealready in production, and in addition the same device package can beused. The problem to be solved in each case is the requirement toperform the processes of stacking and packaging so that the specificparameter-to-be-monitored by the MEMS under consideration remainsunaffected and the package is shielding the new device effectivelyagainst disturbances of the parameter-to-be-monitored.

For a MEMS of high sensitivity to mechanical and thermal stresses,applicants discovered a solution to the problem of device integrationand protection against mechanical and thermal stress disturbances, whenthey developed a process flow with a unique application of low modulusmaterial and a unique set of process steps to protect thestress-sensitive MEMS device while still utilize standard, low costpackage assembly material and assembly methods practiced in production.

The integration of unique materials and process steps into the materialset and process flow used to assemble standard molded packages, such asQuad Flat No-Lead (QFN) packages, includes a polyimide ring with phobicsurface characteristics deposited on a circuitry chip; low modulussilicone compounds and methods to dispense these compounds undercontrolled conditions to surround hexahedron-shaped MEMS devices on allsides; and controlled component adhesion to polymeric packagingcompounds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross section of an integrated polymeric-packageddevice including a stress-sensitive MEMS device vertically assembly on acircuitry chip attached to a substrate, wherein the MEMS is surroundedby a protective low-modulus material.

FIG. 2 shows a diagram of the process flow in a semiconductor waferfabrication according to the invention.

FIG. 3 displays a portion of a process flow in a semiconductor assemblyfactory according to the invention.

FIG. 4 depicts another portion of a process flow in a semiconductorassembly factory according to the invention.

FIG. 5 shows yet another portion of a process flow in a semiconductorassembly factory according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an exemplary embodiment of the invention, a devicegenerally designated 100. The device includes a semiconductor chip 101with terminals 106. A portion of the surface of chip 101 is suitable forthe attachment of a MEMS device 150. In this exemplary embodiment, theMEMS is stress sensitive. An exemplary chip 101 may square shaped, havea side length of several millimeters, for instance 2.5 mm, and include afully functional integrated circuit. In FIG. 1, chip 101 is attached tothe pad 110 of a leadframe, preferably using an adhesive epoxy-basedpolymeric compound 120. The terminals 106 of chip 101 are connected bybonding wires 181 to respective leads 111 of a metallic leadframe; thepreferred wire metal is a copper alloy, alternatively gold or aluminummay be used. The vertical assembly of MEMS, chip, and portions of theleadframe chip 101 is embedded in an insulating package 190, preferablymade of an epoxy-based molding compound.

In the exemplary device of FIG. 1, the leadframe belongs to the QuadFlat No-Lead (QFN) or in Small Outline No-Lead (SON) families. Theseleadframes are preferably made from a flat sheet of a base metal, whichis selected from a group including copper, copper alloys, aluminum,aluminum alloys, iron-nickel alloys, and Kovar. For many devices, theparallel surfaces of the leadframe base metal are treated to createstrong affinity for adhesion to plastic compound, especially moldingcompounds. As an example, the surfaces of copper leadframes may beoxidized, since copper oxide surfaces are known to exhibit good adhesionto molding compounds. Other methods include plasma treatment (describedbelow) of the surfaces, or deposition of thin layers of other metals onthe base metal surface. As an example for copper leadframes, platedlayers of tin have been used, or a layer of nickel (about 0.5 to 2.0 μmthick) followed by a layer of palladium (about 0.01 to 0.1 μm thick)optionally followed by an outermost layer of gold (0.003 to 0.009 μmthick).

It should be noted that other embodiments may use other types ofleadframes with a chip attachment pad, such as leadframes with elongatedleads, with cantilevered leads, or with frames having one or more padsin a plane offset from the plane of the leads. Still other embodimentsmay use laminated substrates made of insulating material alternatingwith conductive layers. These substrates may have an area suitable forattaching one or more chips, and conductive connections suitable forstitch bonding wires.

As FIG. 1 illustrates, one surface of chip 101 is attached to leadframepad 110 by epoxy-based compound 120, the opposite chip surface includesan area of diameter 151 suitable for vertically attaching MEMS device150 using compound 140. In contrast, however, to the polymeric attachcompound 120 with a modulus of >2 GPa, the compound 140 is made of asilicone compound with a comparatively very low modulus of <10 MPa.

As an example, compound 140 may be a silicone compound commerciallyavailable from Dow Corning Corporation (Corporate Center, Midland,Mich., USA). The compound may be further characterized by low viscosityand thixotropic behavior so that it exhibits weakened constitution whendisturbed and strengthened behavior when left standing. Since themodulus of a material characterizes its strain response to an appliedstress (or pressure), compound 140 has very compliant mechanicalcharacteristics. This feature is essential to protect thestress-sensitive MEMS, since low modulus material does not transmitstress but rather distributes and absorbs stress. Consequently, lowmodulus materials, when applied to a side of a stress-sensitive devicesuch as a MEMS, can protect this device against external stress from thecovered side. The stress-protecting characteristics of material 140 needto be preserved through the silicone polymerization cycle occurringduring the elevated temperature required for the wire-bonding the MEMSterminals (see below).

In order to protect a stress sensitive hexahedron-shaped MEMS on allsides against external stress in conjunction with semiconductor devices,the surrounding cocoon of stress-absorbing and stress-dispersingmaterial has to have a thickness suitable to perform functions inconjunction with semiconductor products. As an example, the material hasto have a thickness permitting the attachment of the MEMS to portions ofthe semiconductor device. As another example, the material has to have athickness to allow the incorporation of arching spans of bonding wires.

FIG. 1 shows that the diameter 151 of the silicone attachment layer ispositioned within the inner diameter 131 of a ring 130 of polyimidematerial. The terminals 106 of chip 101, however, remain outside ring130. Ring 130 is made of a polyimide compound. As described below, theconfiguration of ring 130 as a circle, a rectangle, or any other closedstructure, is patterned from a polyimide layer deposited on asemiconductor (preferably silicon) wafer, which includes a plurality ofchips with integrated circuits (ICs). After patterning, the polyimidering is made non-wettable and repellent to silicone materials byreducing the polyimide surface energy in a process cycle, which includesa first curing cycle followed by an ashing process and then in turn by asecond curing cycle. Materials other than polyimide may be used as longas they have silicone-repellent surface characteristics.

After the polyimide ring is made repellent, the IC chips are singulatedfrom the wafer. Each chip may then be attached to the pad of asubstrate, for instance to the pad 110 of a metallic leadframe as shownin FIG. 1. The adhesive used for the attachment layer 120 is preferablyan epoxy-based polymeric formulation.

In the example of FIG. 1, the stress-sensitive MEMS 150 is stackedvertically on IC chip 101 by low modulus silicone layer 140 so that theattachment is inside diameter 131 of the silicone ring 130; layer 140protects MEMS 150 against stress from the direction of chip 101. Afterthe MEMS attachment, terminals 107 of MEMS 150 can be connected tocontact pads 105 of chip 101 using bonding wires 180. It should be notedthat pads 105 are located inside diameter 131 of silicone ring 130.Thereafter, chip terminals 106 are connected to substrate connectors(such as leadframe leads 111) using bonding wires 181.

As FIG. 1 illustrates, MEMS 150 is surrounded by a glob 160, which alsocovers bonding wires 180 and pads 105 and therefore has a dome-shapedconfiguration. Glob 160 is made of low modulus silicone material,preferably the same silicone material as layer 140. As an example, thesilicone of glob 160 may be a silicone commercially available from DowCorning Corporation. The silicone material is selected so that is hashydrophobic and non-adhesive characteristics towards the polymericmolding formulation, which has been selected as encapsulation compound.The viscosity, modulus, and thixotropic index of the silicone materialis selected so that glob 160 can be dispensed in a precision volume andglob 160 is restricted to the surface area inside the polyimide ring 130and does not bleed out across the ring.

After the dispensing process, the silicone compound of glob 160 ispolymerized in a curing process. Thereafter, glob 160 surrounds thenon-attached five sides of the hexahedron-shaped MEMS device and itsconnecting wire bonds. Glob 160 may have approximately the shape of ahemisphere. Together with the low modulus silicone compound of theattach material, all six sides of the MEMS devices are thus surroundedby low modulus material operable to protect the MEMS against any stressby blunting, dispersing and absorbing external stress arriving at theglob surface.

The device including MEMS 150 stacked on circuit chip 101, which in turnis mounted on substrate 110, needs to be in a rigid and strong package.The embodiment of FIG. 1 depicts an encapsulation material 190 formingthe package for a device 100 belonging to the QFN/SON product families.Other embodiments may belong to different product families. Independentof the actual configuration of the package and the leadframe, it isrequired that package material 190 should not affect the operation ofMEMS 150. As a consequence, this requirement means for thestress-sensitive MEMS of FIG. 1 that package material 190 needs toadhere strongly to all parts packaged inside (to prevent delamination)except to blob 160 surrounding MEMS 150. There are two methods toachieve this goal; the different process flows are discussed below.

FIG. 1 indicates a surface 160 a of glob 160 with a layer 170 on thesurface. Based on the first process flow (described in FIGS. 3 and 4),surface 160 a has been activated by plasma etching and thereaftercovered by layer 170 comprising cured low modulus silicone compound. Thepackaging compound 190 will not adhere to layer 170 made of cured (andthus de-activated) silicone compound.

Based on the second process flow (described in FIGS. 3 and 5), surface160 a constitutes cured (and thus de-activated) low modulus silicone,which will not adhere to the packaging compound 190, and is covered bylayer 170 comprising cured epoxy compound. The packaging compound 190,also an epoxy compound, will adhere to and eventually even merge intolayer 170 and face the non-adhesive cured silicone surface 160 a.

In summary, by either one of these methods, packaging compound 190(preferably a molding compound) will not adhere to the cured glob 160and thus cannot transmit external stress into the glob 160 and to thestress-sensitive MEMS 150. Consequently, MEMS 150 can operateundisturbed by external stress.

Another embodiment of the invention is a method to fabricate a commonpackage for a stress-sensitive micro-electro-mechanical system (MEMS)vertically stacked onto a semiconductor circuitry chip. The method isdescribed as a process flow, which includes processes (FIG. 2) performedin a semiconductor wafer factory and processes (either FIGS. 3 and 4, orFIGS. 3 and 5) performed in a semiconductor assembly factory.

The method starts in a wafer factory by providing a silicon wafer, whichcontains a plurality of integrated circuit (IC) chips and has completedthe front-end processes for fabricating the ICs. The intention is tocouple a discrete IC chip with a stress-sensitive MEMS and unite them ina common package executed so that the IC and the MEMS can operateundisturbed. The process flow begins with the processes listed in FIG.2.

In the first step 201 of the process flow, a semiconductor wafer isprovided, which includes a plurality of chips with integrated circuits(ICs). In process 202, a layer of polyimide material is coated over thesurface of the plurality of chips. Then, the polyimide layer ispatterned to form a polyimide ring on each chip, wherein the rings havean inner diameter greater than the largest linear dimension of the MEMSdevice-to-be-assembled. For the patterning process, a photolithographicmethod is used which employs sequentially the process of spinning-on aphotoresist layer, masking the photoresist, exposing the mask-protectedlayer to irradiation, and developing the layer.

After the patterning, the surface energy of the polyimide rings isreduced in process 203 by subjecting the polyimide materialconsecutively to the processes of curing the polyimide compound for afirst time, then ashing the polyimide compound, and finally curing thepolyimide compound for a second time. The ashing process involves anoxygen plasma and elevated temperatures; alternatively, a hydrogenplasma could be used. The reduced surface energy renders the polyimiderings hydrophobic and repellent to low viscosity and low modulussilicone compounds; silicone material deposited inside the polyimidering will not bleed out across the polyimide rings. The IC chips aresingulated from the wafer in process 204.

In process 301, the discrete chips can be attached to respectiveassembly pads of a substrate in an assembly factory. The assembly padmay be the chip pad of a leadframe, or alternatively the assembly pad ofa laminated substrate. Preferably, the chip is attached to the pad usingan epoxy-based compound, which may be cured in a later process atelevated temperature (for instance during wire bonding).

In order to attach a MEMS device to the circuitry chip and use only asingle and preferably small package for the IC chip and MEMScombination, the MEMS needs to be vertically attached to the IC chip. Inthe process 302 of assembling a MEMS device vertically on an IC chip,the MEMS is attached to the chip surface inside the respective polyimidering using a layer of low modulus silicone material. Such materials arecommercially available, for instance from Dow Corning Corporation(Midland, Mich.). For the low modulus material, an attachment layerthickness between about 20 μm and 30 μm may be satisfactory forprotection against external stress. It is preferred to polymerize thesilicone of the attachment layer before advancing to the wire bondingprocess.

For the process of wire bonding the terminals of the MEMS device torespective contact pads on the chip surface inside the ring, wiresincluding gold or copper may be used. During the bonding operation, thewires are typically spanned with an arch from the MEMS terminals to thechip contact pads.

In conjunction with the wire bonding process for the MEMS, the processof wire bonding the terminals 106 of the circuitry chip to respectivecontact pads of the substrate may be performed. Preferably, the bondingwires are made of copper. In FIG. 1, the chip bonding wires aredesignated 181.

In process 303, a glob is formed by dispensing low-modulus siliconematerial over the MEMS device and the MEMS bonding wires, whilerestricting the glob to the surface area inside the polyimide ring. Dueto the arches of the bonding wires, the glob has a dome-shapedconfiguration. The silicone material of the glob is hydrophobic andnon-adhesive to thermoset molding compounds. Due to its low viscosityand thixotropic index, it can be dispensed in precision volume. Inprocess 304, the silicone glob is cured.

Since the MEMS of the exemplary embodiment of FIG. 1 is stress sensitiveand has to be protected against transmission of stress, and since theMEMS shares the same overall package with the circuitry chip, it isrequired that the packaging compound encapsulating the assembly shouldnot adhere to the surface of the glob covering the MEMS, but needs toadhere reliably to all other surfaces inside the package. This dichotomyof requirements can be achieved by two methodologies.

The first methodology is summarized in FIG. 4. In order to achieve goodadhesion to molding compounds, process 401 subjects all surfaces to aplasma etch. The plasma, involving its gas mixture and power for aprescribed time, preferably operates on cooled surfaces. The plasmaaccomplishes thorough cleaning of the surfaces from adsorbed films,especially water monolayers, thereby freeing up electrical bonds. Inaddition, the plasma induces some roughening of the surfaces. Theseeffects enhance the adhesion to polymeric filler-filled moldingcompounds.

Since the cured silicone of the glob has been affected by the plasma ofprocess 401, the glob needs an additional process so that it effectivelyreturns to its original low modulus characteristic. To that end, a layerof the low-modulus silicone material is dispensed onto the surface ofthe glob in process 402. The nozzle of a syringe releases a controlleddrop of the silicone material, which hits the glob and spreads out intoa layer of not quite uniform thickness. While a major amount of the dropmay remain on the impact location of the glob, some material is bleedingout, resulting in a somewhat non-uniform layer as exemplified by layer170 in FIG. 1. For the dimensions of the example in FIG. 1, a drop mayhave a diameter of approximately 30 μm and create a layer of non-uniformthickness with a height of about 13 μm height at the maximum.

In process 403, the silicone layer is cured. Then, in process 404, theassembly comprising the stress-sensitive MEMS embedded in thestress-blocking glob and vertically attached to the semiconductorcircuitry chip is encapsulated in a polymeric molding compound, togetherwith the portion of the substrate, onto which the chip is attached andwire bonded. The integrated device is thus packaged.

The second methodology is summarized in FIG. 5. After the processes offorming a dome-shaped glob by dispensing a low-modulus silicone materialover the MEMS and the wire spans, and curing the silicone glob,described above in processes 303 and 304, a layer of epoxy compound isdispensed onto the surface of the glob in process 501. The epoxy may bethe same as the molding compound, or it may be a formulation such asCRP-4160G, commercially available from the Sumitomo Corporation, Japan.The layer may be uniform, or it may look similar to layer 170 in FIG. 1;the layer is of a nature so that it may merge with, or may be absorbedinto the polymeric encapsulation compound.

In process 502, the epoxy layer is cured. Then, in process 503, allsurfaces are subjected to a plasma etch in order to achieve reliableadhesion to molding compounds. The plasma, involving its gas mixture andpower for a prescribed time, preferably operates on cooled surfaces. Theplasma accomplishes thorough cleaning of the surfaces from adsorbedfilms, especially water monolayers, thereby freeing up electrical bonds.In addition, the plasma induces some roughening of the surfaces. Theseeffects enhance the adhesion to polymeric filler-filled moldingcompounds.

In process 504, the assembly comprising the stress-sensitive MEMSembedded in the stress-blocking glob and vertically attached to thesemiconductor circuitry chip is encapsulated in a polymeric moldingcompound, together with the portion of the substrate, onto which thechip is attached and wire bonded. The integrated device is thuspackaged.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the invention applies to products using anytype of semiconductor chip, discrete or integrated circuit, and thematerial of the semiconductor chip may comprise silicon, silicongermanium, gallium arsenide, gallium nitride, or any other semiconductoror compound material used in integrated circuit manufacturing.

As another example, the invention applies to MEMS having parts movingmechanically under the influence of an energy flow (acoustic, thermal,or optical), a pressure, temperature or voltage difference, or anexternal force or torque. Certain MEMS with a membrane, plate or beamcan be used as a pressure sensor (for instance microphone and speaker),inertial sensor (for instance accelerometer), or capacitive sensor (forinstance strain gauge and RF switch); other MEMS operate as movementsensors for displacement or tilt; bimetal membranes work as temperaturesensors.

It is therefore intended that the appended claims encompass any suchmodifications or embodiment.

We claim:
 1. An integrated circuit (IC) package comprising: a deviceattached to a surface of a substrate; a first material covering thedevice; and a second material covering the first material and portionsof the substrate, the first material having a different modulus than thesecond material; and a structure of polyimide on a surface of thesubstrate and surrounding the device.
 3. The IC package of claim 1,wherein the substrate is a semiconductor chip.
 4. The IC package ofclaim 3, wherein the device is electrically connected to thesemiconductor chip and attached to the substrate via a siliconecompound.
 5. The IC package of claim 3, wherein the semiconductor chipis attached to a pad and electrically connected to leads surrounding thepad.
 6. The IC package of claim 5, wherein the semiconductor chip isattached to the pad by a polymeric compound.
 7. The IC package of claim1, wherein the second material covers portions of the surface of thesubstrate.
 8. The IC package of claim 1, wherein the first material issilicone.
 9. The IC package of claim 1, wherein the second material ismolding compound.
 10. The IC package of claim 1, wherein the firstmaterial has a modulus of less than 10 Mega Pascal.
 11. The IC packageof claim 1, wherein the first material is restricted to portions of thesurface of the substrate inside the structure.
 12. The IC package ofclaim 1, wherein the device is stress sensitive, and wherein the deviceis a MEMS device, and wherein the compound is a silicone compound. 13.The IC package of claim 1, wherein the structure is a closed structurefrom a top view of the IC package.
 14. An integrated circuit (IC)package comprising: a substrate including leads and a pad; asemiconductor chip attached to the pad via a polymeric compound, thesemiconductor chip including first and second terminals, the secondterminals wire bonded to the leads and the semiconductor chip; a deviceattached to a surface of the semiconductor chip, terminals of the deviceelectrically connected to the first terminals; a first material coveringthe device, and portions of the surface of the semiconductor chip; asecond material covering a surface of the first material, the firstmaterial including a different modulus than the second material; and amolding compound covering a surface of the second material, and portionsof the semiconductor chip.
 15. The IC package of claim 14, wherein thefirst and second materials are low modulus silicone materials with amodulus of less than 10 Mega Pascal.
 16. The IC package of claim 14,wherein the molding compound is an epoxy-based thermoset moldingformulation.
 17. The IC package of claim 14, wherein the device isstress sensitive.
 18. The IC package of claim 14 further comprising astructure of polyimide on the surface of the semiconductor chipsurrounding the device and the first terminals.
 19. The IC package ofclaim 18, wherein the closed structure has a shape of one of a circleand a rectangle.
 20. The IC package of claim 18, wherein the firstmaterial is restricted to portions of the surface of the semiconductorchip inside the structure, and wherein the structure is a closedstructure.